In olefin epoxidation, a feed containing an olefin and oxygen is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contain olefin oxide and, typically, unreacted feed and combustion products.
The olefin oxide may be reacted with water to form a 1,2-diol, with carbon dioxide to form a 1,2-carbonate, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-carbonates, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, carbon dioxide, an alcohol, or an amine.
Olefin epoxidation catalysts typically comprise a silver component, usually with one or more additional elements deposited therewith, on a carrier. U.S. Pat. No. 4,766,105 discloses an ethylene oxide catalyst comprising silver, alkali metal, rhenium and a rhenium co-promoter selected from sulfur, molybdenum, tungsten, chromium and mixtures thereof supported on a carrier. The ethylene oxide catalyst described in U.S. Pat. No. 4,766,105 provides an improvement in one or more catalytic properties.
The catalyst performance may be assessed on the basis of selectivity, activity and stability of operation. The selectivity is the fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin converted normally decreases with time and to maintain a constant level of olefin oxide production the temperature of the reaction may be increased.
The selectivity determines to a large extent the economical attractiveness of an epoxidation process. For example, one percent improvement in the selectivity of the epoxidation process can substantially reduce the yearly operating costs of a large scale ethylene oxide plant. Further, the longer the activity and selectivity can be maintained at acceptable values, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the selectivity, activity, and maintenance of the selectivity and activity (i.e., stability) over long periods yield substantial dividends in terms of process efficiency.
In addition, since ethylene oxide catalysts have a limited commercial life (typically 1 to 2 years) the catalyst needs to be exchanged for a fresh batch at that time resulting in lost plant production. Also replacement of the catalyst is expensive as the raw materials like silver are expensive. Extending catalyst life is therefore very attractive from the economic point of view.
One of the reasons for activity and selectivity decline is loss of silver surface area by the mechanism of silver sintering. Eliminating or slowing down silver sintering would extend economic life of the catalyst. This reduction in silver sintering is what can be achieved by practice of the presently claimed invention.